Sharing interesting advancements, issues and challenges in Optics and Photonics

Archive for December, 2012

Two people Malte Gather (Harvard Medical School) and Seok Hyun Yun (MIT) are now making biological cells to emit. Yes, they have demonstrated LASER from biological cells. In the paper published on 12 June 2011 in the journal of Nature Photonics, titled “Single Cell Biological Lasers”, they inform us about the discovery in the following words.

” … lasing has so far relied on artificial or engineered optical gain materials, such as doped crystals, semiconductors, synthetic dyes and purified gases. Here, we show that fluorescent proteins in cells are a viable gain medium for optical amplification, and report the first successful realization of biological cell lasers based on green fluorescent protein (GFP). We demonstrate in vitro protein lasers using recombinant GFP solutions and introduce a laser based on single live cells expressing GFP. On optical pumping with nanojoule/nanosecond pulses, individual cells in a high-Q microcavity produce bright, directional and narrowband laser emission, with characteristic longitudinal and transverse modes. Lasing cells remained alive even after prolonged lasing action. Light amplification and lasing from and within biological systems pave the way to new forms of intracellular sensing, cytometry and imaging.”

We are aware of the difference between a generic light and a laser. It’s just like a crowd of generic people and a crowd of clones. The photons making up a laser are identical copies of each other having same color, direction and phase, thereby producing a monochromatic, directional and coherent beam of light. The phenomenon of stimulated emission is necessary for the production of laser. When a molecule is in excited state, it has some extra energy. The molecule can relax by emitting the extra energy in the form of photon which further excites the interacting molecules. These excited molecules relax, omitting more photons and the process carries on till a heavy population of such photons is achieved. These photons are kept in between two mirrors to build up the desired laser beam.

Gather and Yun employed Green Fluorescent Protein (GFP), a specific protein produced in some cells, to demonstrate lasing effect. GFP is excitable by light and can be used to produce single cell laser. It was discovered in 1962 in a species of bio-luminescent jellyfish and it was after thirty years that scientist found out to use GFP to image cellular structures by genetically modifying the cells. The beautiful images of the cellular anatomy we have today, are courtesy to this GFP. It is the same protein which resulted in Nobel Prize in Chemistry in 2008.

The following demonstration of the first bionic laser represents the shrinking of the gap between the biological and physical communities. Interesting questions arise after this discovery. If lasers can be generated inside a body, it would possibly change the current method of medical diagnosis and treatment. Plasmonic nanoparticles and nano-antennas with tailored optical properties may provide the required cavity for the lasers. The environment sensitive nature of these lasers may result in obtaining more accurate knowledge about the performance, functioning and structure of cells and proteins. It also revealed and interesting fact that the lasers cannot be just optoelectronic based devices. These may be integrated into any living organism.

In this story, there are two protagonists: A Photon and an Electron. The signal/optical communication devices uses Photons (light) and signal processing devices uses Electrons. The conversion of optics into electronics and vice versa slows down the effective communication. Recently, a new transistor has been proposed and experimentally demonstrated by Leonid V. Butov and Arthur C. Gossard (University of California) which processes signals by emitting light.

Signal Processing is mainly carried out before transmitting and receiving any information. It is mainly done using semi-conductor integrated circuits. These miniature integrated circuits are built using transistors which currently uses electrons for signal processing. These electrons and photons don’t interact directly with each other. This presents a major bottle neck in the modern day communication and signal processing. The direct use of light, without its conversion, would speed up both computation and communication.

The transistor proposed is based on Gallium Arsenide (GaAs) and processes signals using indirect EXCITONS instead of electrons. These excitons are controlled by gate electrodes just like in silicon transistors (standard field effect transistors i.e. FETs) and can be easily coupled with the photons. This results in faster signal transmission to other optically connected on chip and off chip devices. The computation power advantage is not great as compared to the communication one.

Excitons are electron-hole pairs, bound by the attractive force between negatively charged electrons and positively charged holes. Because of this force, excitons tend to recom­bine fast, releasing a flash of light. The lifetime can be increased by up to ten microseconds when confining electrons and holes in spatially separated layer forming and indirect exciton. The excitons exhibits stable characteristics at low temperature operation (below 40K) but dissociate easily at higher temperatures. The real time applications require stable operation at room temperature and above. This question still needs to be addressed. There is no inorganic semiconductor material available so far that allows stable exciton population at room temperature. Although materials such as ZnSe, CdTe and GaN may survive exciton populations at room temperature but that requires extremely narrow spatial separation between electron hole pairs of the excitons. This stable operation of excitons remains a bottle neck for the fabrication of high quality exciton based ICs (EXICs).

The processing of excitons require transfer of energy before their decay. This limits the number of transistors integrated on a chip which is a crucial condition for computation. The coupling excitons don’t have a long lifetime (few nanoseconds) and a small propagation distance. The excitons having larger lifetime reveals poor coupling with light. This results in another obstacle in the fabrication of real time operating devices.

The success of this technology depends on how these open questions are to be addressed. Optical Exciton based transistors can be a reality and a paradigm shift if we would be able to find materials and methods for room temperature operation. It could easily pave the way for technological revolution. All I would say is Conventional Solid-State Optoelectronics still has a huge intrinsic potential for further development.

Extracted and Summarized from “Will Excitonic Circuits Change Our Lives?” in Optics and Photonics Focus published in August 2008

The year marks the silver jubilee of Photonic Crystals. The photonic crystals has transformed science and technology by developing new areas which lies at the boundary of condensed matter physics and photonics. Areas such as meta-materials, have been born out of this synergy, striking out in new directions and flourishing in their own right. Last twenty five years has seen numerous of improvements in this area.

The credit goes to two key people Eli Yablonovitch (Bell Labs) and Sajeev John (Princeton University), who tried to address the simple problems of rigorous suppression of spontaneous emission of excited atoms inside a dielectric cavities and the observing Anderson Localization (absence of wave transport in disordered medium) using classical electromagnetic waves in non-dissipative systems, respectively. Interestingly, the solution they both proposed was to design a three dimensional periodic dielectric structure (spatial periodic) in which the complete electromagnetic bandgap opens up in the photon dispersion relation.

The initial work by both of them was theoretical in nature and remained unnoticed for a few years until some initial attempts were made to develop a three-dimensional photonic bandgaps. At this the time the community in this area became aware of the implications and the true potential of this concept. This resulted in a good amount of activity and interesting tangible output.

Furthermore, it has been recognized that periodic nano-structures are abundant in the animal world, where they are responsible for the bright, iridescent colors (particularly blue) found on the skin, wings or feathers of tropical fishes, hummingbirds, moths, beetles, butterflies and peacocks, among other species.

The realization of truly three dimensional photonic crystals, particularly at optical wavelengths, has proved persistently challenging owing to the intricacy of periodically nano-sculpturing a material in all three spatial dimensions. This is a limit at this moment at the technological impact of these structures. With continuous advances in the synthesis, fabrication and self-assembly of photonic nano-materials it is thought that these challenges will eventually be overcomed.

Extracted from Early Lights in Nature Materials |11|95|(2012)| doi:10.1038/nmat3519|Published online on 23 November 2012